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We once thought that quantum effects were confined to the realm of the tiny, but we are now discovering that they can reach into the macroscopic world that we inhabit. The latest evidence comes from metallic particles the size of some viruses, which have just broken the record for the most macroscopic object to ever exhibit quantumness similar to Schrödinger’s cat.

In 1935, Erwin Schrödinger imagined a cat in a quantum superposition of being both dead and alive to underscore the absurdity of quantum mechanics as a theory of everything. In this special state, it would be impossible to tell whether the cat is dead or alive without interacting with it, which led Schrödinger to suggest that the animal is in an unsettling, mixed state of the two. As the world doesn’t actually contain cats that are simultaneously alive and dead, researchers have since adopted the view that, past a certain size, objects lose their quantumness – and hence their ability to embody quantum superpositions – because of disturbances from their environment, or “decoherence”.

But exactly what that size might be, or whether this line between the quantum and the classical worlds absolutely must exist at all, remains an open question. at the University of Vienna in Austria and his colleagues have now moved the boundary further into the macroscopic realm than ever before.

“Standard quantum mechanics does not state any limits; it doesn’t say it stops working at this mass or this size or at this superposition distance,” says Pedalino. “We don’t know if there might be any fundamental limit or new physics that is connected to the mass or the size [of an object], it’s a question that we have to settle by measurement and experiments.”

He and his colleagues performed an with sodium nanoparticles. In such experiments, researchers can diagnose whether an object is in a quantum superposition state by looking for a very specific signal when it is aimed at a detector.

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This experiment is equivalent to sending light through two narrow parallel slits and capturing what comes out of them on a screen. Here, the screen shows an “interference pattern” consisting of alternating light and dark stripes, because the light waves from each slit clash with each other, either strengthening one another when they meet crest-to-crest, or weakening each other when they meet crest-to-trough. A particle in a quantum superposition state does something similar, because each of the two states it seemingly embodies acts like a “matter wave” and clashes and interferes with the other.

Pedalino’s team captured an interference pattern for sodium nanoparticles, each of which contained more than 7000 atoms. The nanoparticles were each in a Schrödinger’s cat-like mix of two positions that were separated by a distance about 16 times the size of the particle. This means we can think of each nanoparticle as being a fuzzy cloud of probabilities spread out across a distance much larger than the nanoparticle itself.

Pedalino says that “big” in quantum terms isn’t just about the size or mass of an object, because the spread between the superposed states and how long the superposition lasts despite decoherence also matter. As such, physicists prefer to use the term “macroscopicity”, which quantifies how much an object tests the limits of quantum mechanics. Coming in with a macroscopicity score of 15.5, his team’s experiment established a new record.

“Reaching a macroscopicity of 15.5 indicates an approximately 10-times increase in the ‘size’ of previously observed effects, thus pushing the validity range of quantum mechanics to systems the size of a large virus. This is a remarkable finding,” says at ETH Zürich in Switzerland.

“It is an impressive accomplishment,” says at the University of Ljubljana in Slovenia.

at the University of Siegen in Germany says that the new work represents not only a technological advance – as nanoparticles must be kept in ultra-high vacuum and significantly slowed and cooled down to be put into a quantum superposition state that can resist decoherence – but will also inform theoretical work on why we don’t see quantum effects in our everyday life.

Several such theories exist, but experiments like the new one are narrowing the regime in which they could be valid. “If there is any modification of quantum theory towards the macroscale, we must keep coming up with new ideas [for] how to observe quantum superpositions with even heavier objects and on longer time scales,” he says.

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Despite the technical difficulty of the experiment – Pedalino says it took more than two years to obtain an interference pattern with the nanoparticles – all the researchers say they expect that future years will see the macroscopicity record broken yet again, possibly showing quantum effects on objects with a macroscopicity score hundreds of times larger.

Kaltenbaek says that there is a practical upshot to this, as preserving the quantumness of macroscopic objects could become useful for developing quantum technologies, such as for simulation and computation. For Pedalino, a major future goal is to repeat the experiment with large biological objects, like viruses, at scales comparable to and possibly beyond today’s metal nanoparticle.

Here, their interference patterns could be a highly sensitive probe for investigating subtle forces acting on the objects – forces that are otherwise difficult to measure, or even inaccessible, with conventional techniques, he says.